Entropy in the Universe

25 January, 2020

If you click on this picture, you’ll see a zoomable image of the Milky Way with 84 million stars:



But stars contribute only a tiny fraction of the total entropy in the observable Universe. If it’s random information you want, look elsewhere!

First: what’s the ‘observable Universe’, exactly?

The further you look out into the Universe, the further you look back in time. You can’t see through the hot gas from 380,000 years after the Big Bang. That ‘wall of fire’ marks the limits of the observable Universe.

But as the Universe expands, the distant ancient stars and gas we see have moved even farther away, so they’re no longer observable. Thus, the so-called ‘observable Universe’ is really the ‘formerly observable Universe’. Its edge is 46.5 billion light years away now!

This is true even though the Universe is only 13.8 billion years old. A standard challenge in understanding general relativity is to figure out how this is possible, given that nothing can move faster than light.

What’s the total number of stars in the observable Universe? Estimates go up as telescopes improve. Right now people think there are between 100 and 400 billion stars in the Milky Way. They think there are between 170 billion and 2 trillion galaxies in the Universe.

In 2009, Chas Egan and Charles Lineweaver estimated the total entropy of all the stars in the observable Universe at 1081 bits. You should think of these as qubits: it’s the amount of information to describe the quantum state of everything in all these stars.

But the entropy of interstellar and intergalactic gas and dust is about ten times more the entropy of stars! It’s about 1082 bits.

The entropy in all the photons in the Universe is even more! The Universe is full of radiation left over from the Big Bang. The photons in the observable Universe left over from the Big Bang have a total entropy of about 1090 bits. It’s called the ‘cosmic microwave background radiation’.

The neutrinos from the Big Bang also carry about 1090 bits—a bit less than the photons. The gravitons carry much less, about 1088 bits. That’s because they decoupled from other matter and radiation very early, and have been cooling ever since. On the other hand, photons in the cosmic microwave background radiation were formed by annihilating
electron-positron pairs until about 10 seconds after the Big Bang. Thus the graviton radiation is expected to be cooler than the microwave background radiation: about 0.6 kelvin as compared to 2.7 kelvin.

Black holes have immensely more entropy than anything listed so far. Egan and Lineweaver estimate the entropy of stellar-mass black holes in the observable Universe at 1098 bits. This is connected to why black holes are so stable: the Second Law says entropy likes to increase.

But the entropy of black holes grows quadratically with mass! So black holes tend to merge and form bigger black holes — ultimately forming the ‘supermassive’ black holes at the centers of most galaxies. These dominate the entropy of the observable Universe: about 10104 bits.

Hawking predicted that black holes slowly radiate away their mass when they’re in a cold enough environment. But the Universe is much too hot for supermassive black holes to be losing mass now. Instead, they very slowly grow by eating the cosmic microwave background, even when they’re not eating stars, gas and dust.

So, only in the far future will the Universe cool down enough for large black holes to start slowly decaying via Hawking radiation. Entropy will continue to increase… going mainly into photons and gravitons! This process will take a very long time. Assuming nothing is falling into it and no unknown effects intervene, a solar-mass black hole takes about 1067 years to evaporate due to Hawking radiation — while a really big one, comparable to the mass of a galaxy, should take about 1099 years.

If our current most popular ideas on dark energy are correct, the Universe will continue to expand exponentially. Thanks to this, there will be a cosmological event horizon surrounding each observer, which will radiate Hawking radiation at a temperature of roughly 10-30 kelvin.

In this scenario the Universe in the very far future will mainly consist of massless particles produced as Hawking radiation at this temperature: photons and gravitons. The entropy within the exponentially expanding ball of space that is today our ‘observable Universe’ will continue to increase exponentially… but more to the point, the entropy density will approach that of a gas of photons and gravitons in thermal equilibrium at 10-30 kelvin.

Of course, it’s quite likely that some new physics will turn up, between now and then, that changes the story! I hope so: this would be a rather dull ending to the Universe.

For more details, go here:

• Chas A. Egan and Charles H. Lineweaver, A larger estimate of the entropy of the universe, The Astrophysical Journal 710 (2010), 1825.

Also read my page on information.


Ordovician Meteor Event

25 September, 2019

About 1/3 of the meteorites hitting Earth today come from one source: the L chondrite parent body, an asteroid 100–150 kilometers across that was smashed in an impact 468 million years ago. This was biggest asteroid collision in the last 3 billion years!

Here is an L-chondrite:

A chondrite is a stony, non-metallic meteorite that was formed form small grains of dust present in the early Solar System. They are the most common kind of meteorite—and the three most common kinds, each with its own somewhat different chemical composition, seem to come from different asteroids.

L chondrites are named that because they are low in iron. Compared to other chondrites, a lot of L chondrites have been heavily shocked—evidence that their parent body was catastrophically disrupted by a large impact.

It seems that roughly 500,000 years after this event, lots of meteorites started hitting Earth: this is called the Ordovician meteor event. Big craters from that event still dot the Earth! Here are some in North America:

Number 3 is the Rock Elm Disturbance, created when a rock roughly 170 meters in diameter slammed into what’s now Wisconsin:

It doesn’t look like much now, but imagine what it must have been like! The crater is about 6 kilometers across. It features intensely fractured quartz grain and a faulted rim.

It seems these big L-chondrite meteors hit the Earth roughly in a line:

Of course the continents didn’t look like this when the meteor hit, about 467.5 million years ago.

One big question is: was the Ordovician meteor event somehow connected to the giant increase in biodiversity during the Ordovician? Here’s a graph of biodiversity over time:

The Cambrian explosion gets all the press, but in terms of the sheer number of new families the so-called Ordovician radiation was bigger. Most animal life was undersea at the time. This is when coral reefs and other complex ocean ecosystems came into being!

There are lots of theories that try to explain the Ordovician radiation. For example, the oxygen concentration in the atmosphere and ocean soared right before the start of the Ordovician period. More than one of these theories could be right. But it’s interesting to think about the possible influence of the Ordovician meteor event.

There were a lot of meteor impacts, but the dust may have been more important. Right now, extraterrestrial dust counts for just 1% of all dust in the Earth’s atmosphere. In the Ordovician, the amount of extraterrestial dust was 1,000 – 10,000 times greater, due to the big smash-up in the asteroid belt! This may have caused the global cooling we see in that period. The Ordovician started out hot, but by the end there were glaciers.

How could this increase biodiversity? The “intermediate disturbance hypothesis” says that biodiversity increases under conditions of mild stress. Some argue this explains the Ordovician radiation.

I’d say this is pretty iffy. But it’s sure interesting! Read more here:

• Birger Schmitz et al., An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body, Science Advances, 18 September 2019.

Another fun question is: where are the remains of the L chondrite parent body? Could they be the asteroids in the Flora family?


Voyager 1

3 September, 2017


Launched 40 years ago, the Voyagers are our longest-lived and most distant spacecraft. Voyager 2 has reached the edge of the heliosphere, the realm where the solar wind and the Sun’s magnetic field live. Voyager 1 has already left the heliosphere and entered interstellar space! A new movie, The Farthest, celebrates the Voyagers’ journey toward the stars:

What has Voyager 1 been doing lately? I’ll skip its amazing exploration of the Solar System….

Leaving the realm of planets

On February 14, 1990, Voyager 1 took the first ever ‘family portrait’ of the Solar System as seen from outside. This includes the famous image of planet Earth known as the Pale Blue Dot:

Soon afterwards, its cameras were deactivated to conserve power and computer resources. The camera software has been removed from the spacecraft, so it would now be hard to get it working again. And here on Earth, the software for reading these images is no longer available!

On February 17, 1998, Voyager 1 reached a distance of 69 AU from the Sun — 69 times farther from the Sun than we are. At that moment it overtook Pioneer 10 as the most distant spacecraft from Earth! Traveling at about 17 kilometers per second, it was moving away from the Sun faster than any other spacecraft. It still is.

That’s 520 million kilometers per year — hard to comprehend. I find it easier to think about this way: it’s 3.6 AU per year. That’s really fast… but not if you’re trying to reach other stars. It will take 20,000 years just to go one light-year.

Termination shock

As Voyager 1 headed for interstellar space, its instruments continued to study the Solar System. Scientists at the Johns Hopkins University said that Voyager 1 entered the termination shock in February 2003. This is a bit like a ‘sonic boom’, but in reverse: it’s the place where the solar wind drops to below the speed of sound. Yes, sound can move through the solar wind, but only sound with extremely long wavelengths — nothing you can hear.

Some other scientists expressed doubt about this, and the issue wasn’t resolved until other data became available, since Voyager 1’s solar-wind detector had stopped working in 1990. This failure meant that termination shock detection had to be inferred from the other instruments on board. We now think that Voyager 1 reached the termination shock on December 15, 2004 — at a distance of 94 AU from the Sun.

Heliosheath

In May 2005, a NASA press release said that Voyager 1 had reached the
heliosheath
. This is a bubble of stagnant solar wind, moving below the speed of sound. It’s outside the termination shock but inside the heliopause, where the interstelllar wind crashes against the solar wind.

On March 31, 2006, amateur radio operators in Germany tracked and received radio waves from Voyager 1 using a 20-meter dish. They
checked their data against data from the Deep Space Network station in Madrid, Spain and yes — it matched. This was the first amateur tracking of Voyager 1!

On December 13, 2010, the the Low Energy Charged Particle device
aboard Voyager 1 showed that it passed the point where the solar wind flows away from the Sun. At this point the solar wind seems to turn sideways, due to the push of the interstellar wind. On this date, the spacecraft was approximately 17.3 billion kilometers from the Sun, or 116 AU.

In March 2011, Voyager 1 was commanded to change its orientation to measure the sideways motion of the solar wind. How? I don’t know. Its solar wind detector was broken.

But anyway, a test roll done in February had confirmed the spacecraft’s ability to maneuver and reorient itself. So, in March it rotated 70 degrees counterclockwise with respect to Earth to detect the solar wind. This was the first time the spacecraft had done any major maneuvering since the family portrait photograph of the planets was taken in 1990.

After the first roll the spacecraft had no problem in reorienting itself with Alpha Centauri, Voyager 1’s guide star, and it resumed sending transmissions back to Earth.

On December 1, 2011, it was announced that Voyager 1 had detected the first Lyman-alpha radiation originating from the Milky Way galaxy. Lyman-alpha radiation had previously been detected from other galaxies, but because of interference from the Sun, the radiation from the Milky Way was not detectable.

Puzzle: What the heck is Lyman-alpha radiation?

On December 5, 2011, Voyager 1 saw that the Solar System’s magnetic field had doubled in strength, basically because it was getting compressed by the pressure of the interstellar wind. Energetic particles originating in the Solar System declined by nearly half, while the detection of high-energy electrons from outside increased 100-fold.

Heliopause and beyond

In June 2012, NASA announced that the probe was detecting even more charged particles from interstellar space. This meant that it was getting close to the heliopause: the place where the gas of interstellar space crashes into the solar wind.

Voyager 1 actually crossed the heliopause in August 2012, although it took another year to confirm this. It was 121 AU from the Sun.



What’s next?

In about 300 years Voyager 1 will reach the Oort cloud, the region of frozen comets. It will take 30,000 years to pass through the Oort cloud. Though it is not heading towards any particular star, in about 40,000 years it will pass within 1.6 light-years of the star Gliese 445.

NASA says:

The Voyagers are destined — perhaps eternally —
to wander the Milky Way.

That’s an exaggeration. The Milky Way will not last forever. In just 3.85 billion years, before our Sun becomes a red giant, the Andromeda galaxy will collide with the Milky Way. In just 100 trillion years, all the stars in the Milky Way will burn out. And in just 10 quintillion years, the Milky Way will have disintegrated, with all the dead stars either falling into black holes or being flung off into intergalactic space.

But still: the Voyagers’ journeys are just beginning. Let’s wish them a happy 40th birthday!

My story here is adapted from this Wikipedia article:

• Wikipedia, Voyager 1.

You can download PDFs of posters commemorating the Voyagers here:

• NASA, NASA and iconic museum honor Voyager spacecraft 40th anniversary, August 30, 2017.




Solar Irradiance Measurements

14 January, 2017

guest post by Nadja Kutz

This blog post is based on a thread in the Azimuth Forum.

The current theories about the Sun’s life-time indicate that the Sun will turn into a red giant in about 5 billion years. How and when this process is going to be destructive to the Earth is still debated. Apparently, according to more or less current theories, there has been a quasilinear increase in luminosity. On page 3 of

• K.-P. Schröder and Robert Connon Smith, Distant future of the Sun and Earth revisited, 2008.

we read:

The present Sun is increasing its average luminosity at a rate of 1% in every 110 million years, or 10% over the next billion years.

Unfortunately I feel a bit doubtful about this, in particular after I looked at some irradiation measurements. But let’s recap a bit.

In the Azimuth Forum I asked for information about solar irradiance measurements . Why I was originally interested in how bright the Sun is shining is a longer story, which includes discussions about the global warming potential of methane. For this post I prefer to omit this lengthy historical survey about my original motivations (maybe I’ll come back to this later). Meanwhile there is an also a newer reason why I am interested in solar irradiance measurements, which I want to talk about here.

Strictly speaking I was not only interested in knowing more about how bright the sun is shining, but how bright each of its ‘components’ is shining. That is, I wanted to see spectrally resolved solar irradiance measurements—and in particular, measurements in the range between the wavelengths of roughly 650 and 950 nanometers.

This led me to the the Sorce mission, which is a NASA sponsored satellite mission, whose website is located at the University of Colorado. The website very nicely provides an interactive interface including a fairly clear and intuitive LISIRD interactive app with which the spectral measurements of the Sun can be studied.

As a side remark I should mention that this NASA mission belongs to the NASA Earth Science mission, which is currently threatened to be scrapped.

By using this app, I found in the 650–950 nanometer range a very strange rise in radiation between 2003 and 2016, which happened mainly in the last 2-3 years. You can see this rise here (click to enlarge):

verlauf774-51linie
spectral line 774.5nm from day 132 to 5073, day 132 starting Jan 24 in 2003, day 5073 is end of 2016

Now, fluctuations within certain spectral ranges within the Sun’s spectrum are not news. Here, however, it looked as if a rather stable range suddenly started to change rather “dramatically”.

I put the word “dramatically” in quotes for a couple of reasons.

Spectral measurements are complicated and prone to measurement errors. Subtle issues of dirty lenses and the like are already enough to suggest that this is no easy feat, so that this strange rise might easily be due to a measurement failure. Moreover, as I said, it looked as this was a fairly stable range over the course of ten years. But maybe this new rise in irradiation is part of the 11 years solar cycle, i.e., a common phenomenon. In addition, although the rise looks big, it may overall still be rather subtle.

So: how subtle or non-subtle is it then?

In order to assess that, I made a quick estimate (see the Forum discussion) and found that if all the additional radiation would reach the ground (which of course it doesn’t due to absorption), then on 1000 square meters you could easily power a lawn mower with that subtle change! I.e., my estimate was 1200 watts for that patch of lawn. Whoa!

That was disconcerting enough to download the data and linearly interpolate it and calculate the power of that change. I wrote a program in Javascript to do that. The computer calculations revealed an answer of 1000 watts, i.e., my estimate was fairly close. Whoa again!

How does this translate to overall changes in solar irradiance? Some increase had already been noticed. NASA wrote 2003 on its webpage:

Although the inferred increase of solar irradiance in 24 years, about 0.1 percent, is not enough to cause notable climate change, the trend would be important if maintained for a century or more.

That was 13 years ago.

I now used my program to calculate the irradiance for one day in 2016 between the wavelengths of 180.5 nm and 1797.62 nm, a quite big part of the solar spectrum, and got the value 627 W/m2. I computed the difference between this and one day in 2003, approximately one solar cycle earlier. I got 0.61 W/m2, which is 0.1% in 13 years, rather then 24 years. Of course this is not an average value, and not really well adjusted to the sun cycle, and fluctuations play a big role in some parts of the spectrum, but well—this might indicate that the overall rate of rise in solar radiation may have doubled. Likewise concerning the question of the sun’s luminosity: for assessing luminosity one would need to take the concrete satellite-earth orbit at the day of measurement into account, as the distance to the sun varies. But still, on a first glance this all appears disconcerting.

Given that this spectral range has for example an overlap with the absorption of water (clouds!), this should at least be discussed.

See how the spectrum splits into a purple and dark red line in the lower circle? (Click to enlarge.)

bergbildtag132tag5073at300kreis
Difference in spectrum between day 132 and 5073

The upper circle displays another rise, which is discussed in the forum.

So concluding, all this looks as if this needs to be monitored a bit more closely. It is important to see whether these rises in irradiance are also displayed in other measurements, so I asked in the Azimuth Forum, but so far have gotten no answer.

The Russian Wikipedia site about solar irradiance unfortunately contains no links to Russian satellite missions (if I haven’t overlooked something), and there exists no Chinese or Indian Wikipedia about solar irradiance. I also couldn’t find any publicly accessible spectral irradiance measurements on the ESA website (although they have some satellites out there). In December I wrote an email to the head of the section solar radiometry of the World Radiation Center (WRC) Wolfgang Finsterle, but I’ve had no answer yet.

In short: if you know about publicly available solar spectral irradiance measurements other than the LISIRD ones, then please let me know.


Shock Breakout

30 March, 2016


Here you can see the brilliant flash of a supernova as its core blasts through its surface. This is an animated cartoon made by NASA based on observations of a red supergiant star that exploded in 2011. It has been sped up by a factor of 240. You can see a graph of brightness showing the actual timescale at lower right.

When a star like this runs out of fuel for nuclear fusion, its core cools. That makes the pressure drop—so the core collapses under the force of gravity.

When the core of a supernova collapses, the infalling matter can reach almost a quarter the speed of light. So when it hits the center, this matter becomes very hot! Indeed, the temperature can reach 100 billion kelvin. That’s 6000 times the temperature of our Sun’s core!

For a supernova less than 25 solar masses, the collapse stops only when the core is compressed into a neutron star. As this happens, lots of electrons and protons become neutrons and neutrinos. Most of the resulting energy is instantly carried away by a ten-second burst of neutrinos. This burst can have an energy of 1046 joules.

It’s hard to comprehend this. It’s what you’d get if you suddenly converted the mass of 18,000 Earths into energy! Astronomers use a specially huge unit with such energies: the foe, which stands for ten to the fifty-one ergs.

That’s 1044 joules. So, a supernova can release 100 foe in neutrinos. By comparison, only 1 or 2 foe come out as light.

Why? Neutrinos can effortlessly breeze through matter. Light cannot! So it takes longer to actually see things happen at the star’s surface—especially since a red supergiant is large. This one was about 500 times the radius of our Sun.

So what happened? A shock wave rushed upward through the star. First it broke through the star’s surface in the form of finger-like plasma jets, which you can see in the animation.

20 minutes later, the full fury of the shock wave reached the surface—and the doomed star exploded in a blinding flash! This is called the shock breakout.

Then the star expanded as a blue-hot ball of plasma.

Here’s how the star’s luminosity changed with time, measured in multiples of the Sun’s luminosity:



Note that while the shock breakout seems very bright, it’s ultimately dwarfed by the luminosity of the expanding ball of plasma. So, KSN2011d was actually one of the first two supernovae for which the shock breakout was seen! For details, read this:

• P. M. Garnavich, B. E. Tucker, A. Rest, E. J. Shaya, R. P. Olling, D. Kasen and A. Villar, Shock breakout and early light curves of Type II-P supernovae observed with Kepler.

A Type II supernova is one that shows hydrogen in its spectral lines: these are commonly formed by the collapse of a star that has run out of fuel in its core, but retains hydrogen in its outer layers. A Type II-P is one that shows a plateau in its light curve: the P is for ‘plateau’. These are more common than the Type II-L, which show a more rapid (‘linear’) decay in their luminosity:




Hard X-Ray Burst

25 February, 2016

I just learned something cool: 0.4 seconds after LIGO saw those gravitational waves on 14 September 2015, a satellite named Fermi detected a burst of X-rays!

• V. Connaughton et al, Fermi GBM observations of LIGO gravitational wave event GW150914.

It lasted one second. It was rather weak (for such things). The photons emitted ranged from 50 keV to 10 MeV in energy, with a peak around 3.5 MeV. The paper calls this event a ‘hard X-ray source’. Wikipedia says photons with an energy over 100 keV deserve the name gamma rays, while those between 10 keV and 100 keV are ‘hard X-rays’. So, maybe this event deserves to be maybe a gamma ray burst. I suppose it’s all just a matter of semantics: it’s not as if there’s any sharp difference between a highly energetic X-ray and a low-energy gamma ray.

Whatever you call it, this event does not appear connected with other previously known objects. It’s hard to tell exactly where it happened. But its location is consistent with what little we know about the source of the gravitational waves.

If this X-ray burst was caused by the same event that created the gravitational waves, that would be surprising. Everyone assumed the gravitational waves were formed by two large black holes that had been orbiting each other for millions or billions of years, slowly spiraling down. In this scenario we don’t expect much electromagnetic radiation when the black holes finally collide.

Perhaps those expectations are wrong. Or maybe—just maybe—both the gravitational waves and X-rays were formed during the collapse of a single very large star! That’s what typically causes gamma ray bursts—we think. But it’s not at all typical—as far as we know—for a large star to form two black holes when it collapses! And that’s what we’d need to get that gravitational wave event: two black holes, which then spiral down and merge into one!

Here’s an analysis of the issue:

• Abraham Loeb, Electromagnetic counterparts to black hole mergers detected by LIGO.

As he notes, the collapsing star would need to have an insane amount of angular momentum to collapse into a dumb-bell shape and form two black holes, each roughly 30 times the mass of our Sun, which then quickly spiral down and collide.

Furthermore, as Tony Wells pointed to me, the lack of neutrinos argues against the idea that this event involved a large collapsing star:

• ANTARES collaboration, High-energy neutrino follow-up search of Gravitational wave event GW150914 with ANTARES and IceCube.

To add to the muddle, another satellite devoted to observing gamma rays, called INTEGRAL, did not see anything:

• V. Savchenko et al, INTEGRAL upper limits on gamma-ray emission associated with the gravitational wave event GW150914.

It will take a while to sort this out.

But luckily, the first gravitational wave burst seen by LIGO was not the only one! Dennis Overbye of the New York Times writes:

Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Dr. Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer. In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up. There are hopes for more in the future, in India and Japan.

So we will know more soon!

For more on Fermi:


Rumors of Gravitational Waves

7 February, 2016

The Laser Interferometric Gravitational-Wave Observatory or LIGO is designed to detect gravitational waves—ripples of curvature in spacetime moving at the speed of light. It’s recently been upgraded, and it will either find gravitational waves soon or something really strange is going on.

Rumors are swirling that LIGO has seen gravitational waves produced by two black holes, of 29 and 36 solar masses, spiralling towards each other—and then colliding to form a single 62-solar-mass black hole!

You’ll notice that 29 + 36 is more than 62. So, it’s possible that three solar masses were turned into energy, mostly in the form of gravitational waves!

According to these rumors, the statistical significance of the signal is supposedly very high: better than 5 sigma! That means there’s at most a 0.000057% probability this event is a random fluke – assuming nobody made a mistake.

If these rumors are correct, we should soon see an official announcement. If the discovery holds up, someone will win a Nobel prize.

The discovery of gravitational waves is completely unsurprising, since they’re predicted by general relativity, a theory that’s passed many tests already. But it would open up a new window to the universe – and we’re likely to see interesting new things, once gravitational wave astronomy becomes a thing.

Here’s the tweet that launched the latest round of rumors:

ligo_tweet_cliff_burgess

For background on this story, try this:

Tale of a doomed galaxy, Azimuth, 8 November 2015.

The first four sections of that long post discuss gravitational waves created by black hole collisions—but the last section is about LIGO and an earlier round of rumors, so I’ll quote it here!


LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about the supermassive black holes that live in the centers of galaxies. I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.

Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”

The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.


Aggressively Expanding Civilizations

5 February, 2016

Ever since I became an environmentalist, the potential destruction wrought by aggressively expanding civilizations has been haunting my thoughts. Not just here and now, where it’s easy to see, but in the future.

In October 2006, I wrote this in my online diary:

A long time ago on this diary, I mentioned my friend Bruce Smith’s nightmare scenario. In the quest for ever faster growth, corporations evolve toward ever faster exploitation of natural resources. The Earth is not enough. So, ultimately, they send out self-replicating von Neumann probes that eat up solar systems as they go, turning the planets into more probes. Different brands of probes will compete among each other, evolving toward ever faster expansion. Eventually, the winners will form a wave expanding outwards at nearly the speed of light—demolishing everything behind them, leaving only wreckage.

The scary part is that even if we don’t let this happen, some other civilization might.

The last point is the key one. Even if something is unlikely, in a sufficiently large universe it will happen, as long as it’s possible. And then it will perpetuate itself, as long as it’s evolutionarily fit. Our universe seems pretty darn big. So, even if a given strategy is hard to find, if it’s a winning strategy it will get played somewhere.

So, even in this nightmare scenario of "spheres of von Neumann probes expanding at near lightspeed", we don’t need to worry about a bleak future for the universe as a whole—any more than we need to worry that viruses will completely kill off all higher life forms. Some fraction of civilizations will probably develop defenses in time to repel the onslaught of these expanding spheres.

It’s not something I stay awake worrying about, but it’s a depressingly plausible possibility. As you can see, I was trying to reassure myself that everything would be okay, or at least acceptable, in the long run.

Even earlier, S. Jay Olson and I wrote a paper together on the limitations in accurately measuring distances caused by quantum gravity. If you try to measure a distance too accurately, you’ll need to concentrate so much energy in such a small space that you’ll create a black hole!

That was in 2002. Later I lost touch with him. But now I’m happy to discover that he’s doing interesting work on quantum gravity and quantum information processing! He is now at Boise State University in Idaho, his home state.

But here’s the cool part: he’s also studying aggressively expanding civilizations.

Expanding bubbles

What will happen if some civilizations start aggressively expanding through the Universe at a reasonable fraction of the speed of light? We don’t have to assume most of them do. Indeed, there can’t be too many, or they’d already be here! More precisely, the density of such civilizations must be low at the present time. The number of them could be infinite, since space is apparently infinite. But none have reached us. We may eventually become such a civilization, but we’re not one yet.

Each such civilization will form a growing ‘bubble’: an expanding sphere of influence. And occasionally, these bubbles will collide!

Here are some pictures from a simulation he did:





As he notes, the math of these bubbles has already been studied by researchers interested in inflationary cosmology, like Alan Guth. These folks have considered the possibility that in the very early Universe, most of space was filled with a ‘false vacuum’: a state of matter that resembles the actual vacuum, but has higher energy density.

A false vacuum could turn into the true vacuum, liberating energy in the form of particle-antiparticle pairs. However, it might not do this instantly! It might be ‘metastable’, like ball number 1 in this picture:

It might need a nudge to ‘roll over the hill’ (metaphorically) and down into the lower-energy state corresponding to the true vacuum, shown as ball number 3. Or, thanks to quantum mechanics, it might ‘tunnel’ through this hill.

The balls and the hill are just an analogy. What I mean is that the false vacuum might need to go through a stage of having even higher energy density before it could turn into the true vacuum. Random fluctuations, either quantum-mechanical or thermal, could make this happen. Such a random fluctuation could happen in one location, forming a ‘bubble’ of true vacuum that—under certain conditions—would rapidly expand.

It’s actually not very different from bubbles of steam forming in superheated water!

But here’s the really interesting Jay Olson noted in his first paper on this subject. Research on bubbles in the inflationary cosmology could actually be relevant to aggressively expanding civilizations!

Why? Just as a bubble of expanding true vacuum has different pressure than the false vacuum surrounding it, the same might be true for an aggressively expanding civilization. If they are serious about expanding rapidly, they may convert a lot of matter into radiation to power their expansion. And while energy is conserved in this process, the pressure of radiation in space is a lot bigger than the pressure of matter, which is almost zero.

General relativity says that energy density slows the expansion of the Universe. But also—and this is probably less well-known among nonphysicists—it says that pressure has a similar effect. Also, as the Universe expands, the energy density and pressure of radiation drops at a different rate than the energy density of matter.

So, the expansion of the Universe itself, on a very large scale, could be affected by aggressively expanding civilizations!

The fun part is that Jay Olson actually studies this in a quantitative way, making some guesses about the numbers involved. Of course there’s a huge amount of uncertainty in all matters concerning aggressively expanding high-tech civilizations, so he actually considers a wide range of possible numbers. But if we assume a civilization turns a large fraction of matter into radiation, the effects could be significant!

The effect of the extra pressure due to radiation would be to temporarily slow the expansion of the Universe. But the expansion would not be stopped. The radiation will gradually thin out. So eventually, dark energy—which has negative pressure, and does not thin out as the Universe expands—will win. Then the Universe will expand exponentially, as it is already beginning to do now.

(Here I am ignoring speculative theories where dark energy has properties that change dramatically over time.)

Jay Olson’s work

Here are his papers on this subject. The abstracts sketch his results, but you have to look at the papers to see how nice they are. He’s thought quite carefully about these things.

• S. Jay Olson, Homogeneous cosmology with aggressively expanding civilizations, Classical and Quantum Gravity 32 (2015) 215025.

Abstract. In the context of a homogeneous universe, we note that the appearance of aggressively expanding advanced life is geometrically similar to the process of nucleation and bubble growth in a first-order cosmological phase transition. We exploit this similarity to describe the dynamics of life saturating the universe on a cosmic scale, adapting the phase transition model to incorporate probability distributions of expansion and resource consumption strategies. Through a series of numerical solutions spanning several orders of magnitude in the input assumption parameters, the resulting cosmological model is used to address basic questions related to the intergalactic spreading of life, dealing with issues such as timescales, observability, competition between strategies, and first-mover advantage. Finally, we examine physical effects on the universe itself, such as reheating and the backreaction on the evolution of the scale factor, if such life is able to control and convert a significant fraction of the available pressureless matter into radiation. We conclude that the existence of life, if certain advanced technologies are practical, could have a significant influence on the future large-scale evolution of the universe.

• S. Jay Olson, Estimates for the number of visible galaxy-spanning civilizations and the cosmological expansion of life.

Abstract. If advanced civilizations appear in the universe with a desire to expand, the entire universe can become saturated with life on a short timescale, even if such expanders appear but rarely. Our presence in an untouched Milky Way thus constrains the appearance rate of galaxy-spanning Kardashev type III (K3) civilizations, if it is assumed that some fraction of K3 civilizations will continue their expansion at intergalactic distances. We use this constraint to estimate the appearance rate of K3 civilizations for 81 cosmological scenarios by specifying the extent to which humanity could be a statistical outlier. We find that in nearly all plausible scenarios, the distance to the nearest visible K3 is cosmological. In searches where the observable range is limited, we also find that the most likely detections tend to be expanding civilizations who have entered the observable range from farther away. An observation of K3 clusters is thus more likely than isolated K3 galaxies.

• S. Jay Olson, On the visible size and geometry of aggressively expanding civilizations at cosmological distances.

Abstract. If a subset of advanced civilizations in the universe choose to rapidly expand into unoccupied space, these civilizations would have the opportunity to grow to a cosmological scale over the course of billions of years. If such life also makes observable changes to the galaxies they inhabit, then it is possible that vast domains of life-saturated galaxies could be visible from the Earth. Here, we describe the shape and angular size of these domains as viewed from the Earth, and calculate median visible sizes for a variety of scenarios. We also calculate the total fraction of the sky that should be covered by at least one domain. In each of the 27 scenarios we examine, the median angular size of the nearest domain is within an order of magnitude of a percent of the whole celestial sphere. Observing such a domain would likely require an analysis of galaxies on the order of a giga-lightyear from the Earth.

Here are the main assumptions in his first paper:

1. At early times (relative to the appearance of life), the universe is described by the standard cosmology – a benchmark Friedmann-Robertson-Walker (FRW) solution.

2. The limits of technology will allow for self-reproducing spacecraft, sustained relativistic travel over cosmological distances, and an efficient process to convert baryonic matter into radiation.

3. Control of resources in the universe will tend to be dominated by civilizations that adopt a strategy of aggressive expansion (defined as a frontier which expands at a large fraction of the speed of the individual spacecraft involved), rather than those expanding diffusively due to the conventional pressures of population dynamics.

4. The appearance of aggressively expanding life in the universe is a spatially random event and occurs at some specified, model-dependent rate.

5. Aggressive expanders will tend to expand in all directions unless constrained by the presence of other civilizations, will attempt to gain control of as much matter as is locally available for their use, and once established in a region of space, will consume mass as an energy source (converting it to radiation) at some specified, model-dependent rate.


Curiosity Meets Martian Dunes

17 January, 2016

In December, the rover Curiosity reached some sand dunes on Mars, giving us the first views of these dunes taken from the ground instead of from above. It’s impressive how the dune seems to shoot straight up from the rocks here!

In fact this slope—the steep downwind slope of one of “Bagnold Dunes” along the northwestern flank of Mount Sharp—is just about 27°. But mountaineers will confirm that slopes always looks steeper than they are.

The wind makes this dune move about one meter per year.

For more, see:

• NASA, NASA Mars rover Curiosity reaches sand dunes, 10 December 2015.

• Jet Propulsion Laboratory, Mastcam telephoto of a Martian dune’s downwind face, 4 January 2016.

• Jet Propulsion Laboratory, Slip face on downwind side of ‘Namib’ sand dune on Mars, 6 January 2016.



Tale of a Doomed Galaxy

8 November, 2015

Part 1



About 3 billion years ago, if there was intelligent life on the galaxy we call PG 1302-102, it should have known it was in serious trouble.

Our galaxy has a supermassive black hole in the middle. But that galaxy had two. One was about ten times as big as the other. Taken together, they weighed a billion times as much as our Sun.

They gradually spiraled in towards each other… and then, suddenly, one fine morning, they collided. The resulting explosion was 10 million times more powerful than a supernova—more powerful than anything astronomers here on Earth have ever seen! It was probably enough to wipe out all life in that galaxy.

We haven’t actually seen this yet. The light and gravitational waves from the disaster are still speeding towards us. They should reach us in roughly 100,000 years. We’re not sure when.

Right now, we see the smaller black hole still orbiting the big one, once every 5 years. In fact it’s orbiting once every 4 years! But thanks to the expansion of the universe, PG 1302-102 is moving away from us so fast that time on that distant galaxy looks significantly slowed down to us.

Orbiting once every 4 years: that doesn’t sound so fast. But the smaller black hole is about 2000 times more distant from its more massive companion than Pluto is from our Sun! So in fact it’s moving at very high speed – about 1% of the speed of light. We can actually see it getting redshifted and then blueshifted as it zips around. And it will continue to speed up as it spirals in.

What exactly will happen when these black holes collide? It’s too bad we won’t live to see it. We’re far enough that it will be perfectly safe to watch from here! But the human race knows enough about physics to say quite a lot about what it will be like. And we’ve built some amazing machines to detect the gravitational waves created by collisions like this—so as time goes on, we’ll know even more.

Part 2



Even before the black holes at the heart of PG 1302-102 collided, life in that galaxy would have had a quasar to contend with!

This is a picture of Centaurus A, a much closer galaxy with a quasar in it. A quasar is huge black hole in the middle of a galaxy—a black hole that’s eating lots of stars, which rip apart and form a disk of hot gas as they spiral in. ‘Hot’ is an understatement, since this gas moves near the speed of light. It gets so hot that it pumps out intense jets of particles – from its north and south poles. Some of these particles even make it to Earth.

Any solar system in Centaurus A that gets in the way of those jets is toast.

And these jets create lots of radiation, from radio waves to X-rays. That’s how we can see quasars from billions of light years away. Quasars are the brightest objects in the universe, except for short-lived catastrophic events like the black hole collisions and gamma-ray bursts from huge dying stars.

It’s hard to grasp the size and power of such things, but let’s try. You can’t see the black hole in the middle of this picture, but it weighs 55 million times as much as our Sun. The blue glow of the jets in this picture is actually X rays. The jet at upper left is 13,000 light years long, made of particles moving at half the speed of light.

A typical quasar puts out a power of roughly 1040 watts. They vary a lot, but let’s pick this number as our ‘standard quasar’.

But what does 1040 watts actually mean? For comparison, the Sun puts out 4 x 1026 watts. So, we’re talking 30 trillion Suns. But even that’s too big a number to comprehend!

Maybe it would help to say that the whole Milky Way puts out 5 x 1036 watts. So a single quasar, at the center of one galaxy, can have the power of 2000 galaxies like ours.

Or, we can work out how much energy would be produced if the entire mass of the Moon were converted into energy. I’m getting 6 x 1039 joules. That’s a lot! But our standard quasar is putting out a bit more power than if it were converting one Moon into energy each second.

But you can’t just turn matter completely into energy: you need an equal amount of antimatter, and there’s not that much around. A quasar gets its power the old-fashioned way: by letting things fall down. In this case, fall down into a black hole.

To power our standard quasar, 10 stars need to fall into the black hole every year. The biggest quasars eat 1000 stars a year. The black hole in our galaxy gets very little to eat, so we don’t have a quasar.

There are short-lived events much more powerful than a quasar. For example, a gamma-ray burst, formed as a hypergiant star collapses into a black hole. A powerful gamma-ray burst can put out 10^44 watts for a few seconds. That’s equal to 10,000 quasars! But quasars last a long, long time.

So this was life in PG 1302-102 before things got really intense – before its two black holes spiraled into each other and collided. What was that collision like? I’ll talk about that next time.

The above picture of Centaurus A was actually made from images taken by three separate telescopes. The orange glow is submillimeter radiation – between infrared and microwaves—detected by the Atacama Pathfinder Experiment (APEX) telescope in Chile. The blue glow is X-rays seen by the Chandra X-ray Observatory. The rest is a photo taken in visible light by the Wide Field Imager on the Max-Planck/ESO 2.2 meter telescope, also located in Chile. This shows the dust lanes in the galaxy and background stars.

Part 3


What happened at the instant the supermassive black holes in the galaxy PG 1302-102 finally collided?

We’re not sure yet, because the light and gravitational waves will take time to get here. But physicists are using computers to figure out what happens when black hole collide!

Here you see some results. The red blobs are the event horizons of two black holes.

First the black holes orbit each other, closer and closer, as they lose energy by emitting gravitational radiation. This is called the ‘inspiral’ phase.

Then comes the ‘plunge’ and ‘merger’. They plunge towards each other. A thin bridge forms between them, which you see here. Then they completely merge.

Finally you get a single black hole, which oscillates and then calms down. This is called the ‘ringdown’, because it’s like a bell ringing, loudly at first and then more quietly. But instead of emitting sound, it’s emitting gravitational waves—ripples in the shape of space!

In the top picture, the black holes have the same mass: one looks smaller, but that’s because it’s farther away. In the bottom picture, the black hole at left is twice as massive.

Here’s one cool discovery. An earlier paper had argued there could be two bridges, except in very symmetrical situations. If that were true, a black hole could have the topology of a torus for a little while. But these calculations showed that – at least in the cases they looked at—there’s just one bridge.

So, you can’t have black hole doughnuts. At least not yet.

These calculations were done using free software called SpEC. But before you try to run it at home: the team that puts out this software says:

Because of the steep learning curve and complexity of SpEC, new users are typically introduced to SpEC through a collaboration with experienced SpEC users.

It probably requires a lot of computer power, too. These calculations are very hard. We know the equations; they’re just tough to solve. The first complete simulation of an inspiral, merger and ringdown was done in 2005.

The reason people want to simulate colliding black holes is not mainly to create pretty pictures, or even understand what happens to the event horizon. It’s to understand the gravitational waves they will produce! People are building better and better gravitational wave detectors—more on that later—but we still haven’t seen gravitational waves. This is not surprising: they’re very weak. To find them, we need to filter out noise. So, we need to know what to look for.

The pictures are from here:

• Michael I. Cohen and Jeffrey D. Kaplan and Mark A. Scheel, On toroidal horizons in binary black hole inspirals, Phys. Rev. D 85 (2012), 024031.

Part 4

Let’s imagine an old, advanced civilization in the doomed galaxy PG 1302-102.

Long ago they had mastered space travel. Thus, they were able to survive when their galaxy collided with another—just as ours will collide with Andromeda four billion years from now. They had a lot of warning—and so do we. The picture here shows what Andromeda will look like 250 million years before it hits.

They knew everything we do about astronomy—and more. So they knew that when galaxies collide, almost all stars sail past each other unharmed. A few planets get knocked out of orbit. Colliding clouds of gas and dust form new stars, often blue giants that live short, dramatic lives, going supernova after just 10 million years.

All this could be handled by not being in the wrong place at the wrong time. They knew the real danger came from the sleeping monsters at the heart of the colliding galaxies.

Namely, the supermassive black holes!

Almost every galaxy has a huge black hole at its center. This black hole is quiet when not being fed. But when galaxies collide, lots of gas and dust and even stars get caught by the gravity and pulled in. This material form a huge flat disk as it spirals down and heats up. The result is an active galactic nucleus.

In the worst case, the central black holes can eat thousands of stars a year. Then we get a quasar, which easily pumps out the power of 2000 ordinary galaxies.

Much of this power comes out in huge jets of X-rays. These jets keep growing, eventually stretching for hundreds of thousands of light years. The whole galaxy becomes bathed in X-rays—killing all life that’s not prepared.

Let’s imagine a civilization that was prepared. Natural selection has ways of weeding out civilizations that are bad at long-term planning. If you’re prepared, and you have the right technology, a quasar could actually be a good source of power.

But the quasar was just the start of the problem. The combined galaxy had two black holes at its center. The big one was at least 400 million times the mass of our Sun. The smaller one was about a tenth as big—but still huge.

They eventually met and started to orbit each other. By flinging stars out the way, they gradually came closer. It was slow at first, but the closer they got, the faster they circled each other, and the more gravitational waves they pumped out. This carried away more energy—so they moved closer, and circled even faster, in a dance with an insane, deadly climax.

Right now—here on Earth, where it takes a long time for the news to reach us—we see that in 100,000 years the two black holes will spiral down completely, collide and merge. When this happens, a huge pulse of gravitational waves, electromagnetic radiation, matter and even antimatter will blast through the galaxy called PG 1302-102.

I don’t know exactly what this will be like. I haven’t found papers describing this kind of event in detail.

One expert told the New York Times that the energy of this explosion will equal 100 million supernovae. I don’t think he was exaggerating. A supernova is a giant star whose core collapses as it runs out of fuel, easily turning several Earth masses of hydrogen into iron before you can say “Jack Robinson”. When it does this, it can easily pump out 1044 joules of energy. So, 100 millon supernovae is 1052 joules. By contrast, if we could convert all the mass of the black holes in PG 1302-102. into energy, we’d get about 1056 joules. So, our expert was just saying that their merger will turns 0.01% of their combined mass into energy. That seems quite reasonable to me.

But I want to know what happens then! What will the explosion do to the galaxy? Most of the energy comes out as gravitational radiation. Gravitational waves don’t interact very strongly with matter. But when they’re this strong, who knows? And of course there will be plenty of ordinary radiation, as the accretion disk gets shredded and sucked into the new combined black hole.

The civilization I’m imagining was smart enough not to stick around. They decided to simply leave the galaxy.

After all, they could tell the disaster was coming, at least a million years in advance. Some may have decided to stay and rough it out, or die a noble death. But most left.

And then what?

It takes a long time to reach another galaxy. Right now, travelling at 1% the speed of light, it would take 250 million years to reach Andromeda from here.

But they wouldn’t have to go to another galaxy. They could just back off, wait for the fireworks to die down, and move back in.

So don’t feel bad for them. I imagine they’re doing fine.

By the way, the expert I mentioned is S. George Djorgovski of Caltech, mentioned here:

• Dennis Overbye, Black holes inch ahead to violent cosmic union, New York Times, 7 January 2015.

Part 5


When distant black holes collide, they emit a burst of gravitational radiation: a ripple in the shape of space, spreading out at the speed of light. Can we detect that here on Earth? We haven’t yet. But with luck we will soon, thanks to LIGO.

LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about supermassive black holes like the ones in the doomed galaxy of my story here! I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.

Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”

The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.

References

To see where the pictures are from, click on them. For more, try this:

• Ravi Mandalia, Black hole binary entangled by gravity progressing towards deadly merge.

The picture of Andromeda in the nighttime sky 3.75 billion years from now was made by NASA. You can see a whole series of these pictures here:

• NASA, NASA’s Hubble shows Milky Way is destined for head-on collision, 31 March 2012.

Let’s get ready! For starters, let’s deal with global warming.